The present invention relates to methods of preparing carbon-based electrodes for use in electrochemical energy storage devices. More particularly, the invention relates to methods that involve precipitating a polymer from solution, extracting solvent from the polymer, drying the polymer, and pyrolyzing the polymer to form a carbon material.
"Rocking chair" or "lithium-ion" batteries employ a carbon-based anode which intercalates lithium. All carbon rocking chair batteries include a carbon insertion anode, a high voltage insertion cathode, and a lithium ion (or sodium ion) conducting solution (e.g., liquid electrolyte with separator, solid ionically conducting electrolyte, solid polymer electrolyte, or gel polymer electrolyte). During charging, lithium (or sodium) ions from an electrolyte are taken up by the carbon anode (i.e., intercalated), and during discharging those same lithium ions are transported from the anode, through the electrolyte, into an insertion cathode. Properties which make such batteries attractive include (1) a high capacity per mass and volume, (2) a large voltage difference between the cathode and anode, and (3) a high stability at both very positive and very negative voltages. Various carbon-based materials have been proposed for use as the anode, but most of these materials have one or more disadvantages as discussed below.
U.S. Pat. No. 4,423,125 entitled "Ambient Temperature Rechargeable Battery", issued to Basu, describes battery including a lithium intercalated graphite anode and an organic electrolyte. Unfortunately, the described graphite anode is unsuitable for use with many desirable electrolytes (particularly propylene carbonate based electrolytes, and electrolytes having certain salts such as LiClO.sub.4 and LiPF.sub.6). It is believed that the solvents or salts of such electrolytes are inserted into the graphite ring planes, or interplanar regions, resulting in excessive structural stress, exfoliation of the material, and poor cycleability. In addition, lithium is believed to diffuse much slower in graphite than in less ordered carbons. Furthermore, the high quality, highly ordered synthetic graphite needed to ensure high intercalation levels and good cycleability can be difficult and costly to produce. This is because such materials must be produced by pyrolysis of organic compounds at rather high temperatures (&gt;2500.degree. C.), which adds to the cost of the material. Finally, suitable graphite precursors are limited to a relatively small number of organic molecules and polymers.
Partially-graphitic, less ordered, and amorphous carbon anodes have been proposed for use as lithium intercalation electrodes in U.S. Pat. No. 5,028,500 entitled "Carbonaceous Electrodes for Lithium Cells" and issued to Fong et al. That patent describes carbon anodes which contain (1) highly graphitized carbon, (2) substantially lower ordered carbon, or (3) both highly graphitized and less graphitized phases. In addition, the patent notes that carbon black may be used as an additive in some anodes.
To minimize an observed irreversible capacity loss (a parasitic process which decreases the cell's potential capacity), U.S. Pat. No. 5,028,500 specifies that the surface area of the carbon particles which make up the electrode should have a surface area of less than 10 m.sup.2 /gm, with the most preferable range being below 2 m.sup.2 /gm. Although carbon particles in this size range may be desirable for use in electrodes, the patent does not address the procedure by which they are formed from pyrolyzed carbon. Certain difficulties are inherent in many such procedures. For example, it can be difficult to produce carbon particles of a desired size range from coke or other conventional starting material. Further, when such materials are convened to particles, the process typically produces many reactive "fresh" surfaces which can be readily oxidized or otherwise reacted soon after they are exposed, thus degrading the carbon's ability to intercalate and increasing the irreversible capacity loss.
Two other types of carbon-based lithium intercalation electrode are proposed in U.S. Pat. No. 5,358,802 (U.S. Ser. No. 08/041,507, attorney docket no. IL-9060), issued on Oct. 25, 1994, entitled "Doping of Carbon Foams for Use in Energy Storage Devices," naming Mayer, Pekala, Morrison, and Kaschmitter as inventors, and incorporated herein by reference for all purposes. Specifically, this patent describes as suitable intercalation anodes (1) doped carbon foams formed from crosslinked, e.g., resorcinol/formaldehyde aquagels, and (2) doped carbon materials produced by thermally induced precipitation of polyacrylonitrile ("PAN") from dimethyl sulfoxide ("DMSO") solutions. While these materials, especially the second material, have an improved morphology, further improvements in their processing techniques would be desirable. For example, the doped carbon material from precipitated PAN has been prepared with an expensive freeze drying step.
Accordingly, there is a need for further improvement in carbon-based lithium intercalation electrodes and methods of manufacturing such electrodes.